MEMS device with particle filter and method of manufacture

Abstract

A MEMS sensor with a media access opening in its carrier board. The MEMS sensor has an integrally filter mesh closing the media access opening. The mesh can be applied in unstructured form over the whole surface of the carrier board. Then, a structuring is performed to produce preferably at the same time a perforation forming the filter mesh.

Claims

1. A method of manufacturing a MEMS sensor, the MEMS sensor comprising a carrier board with a media access opening, a MEMS chip mounted on the carrier board above the media access opening, and a film applied on a surface of the carrier board facing the MEMS chip and covering the media access opening, the method comprising the steps of: providing the carrier board having the media access opening; applying the film onto a top surface of the carrier board completely covering the media access opening, wherein the film consists of an organic material and is mounted on the carrier board by gluing; providing perforations in a perforation area of the film, the perforation area extending at least partly over an area of the media access opening; mounting the MEMS chip having a surface with a sensing area on the top surface of the carrier board such that a medium to be sensed can reach the sensing area through the media access opening, and wherein the film includes a polymer in a B stage acting as film and adhesive, the film is applied by lamination, and the film is cured after applying.

2. The method of claim 1, wherein the film is applied to the entire top surface of the carrier board wherein the film, after applying, is structured to be restricted in area to the area of the media access opening on the top surface plus a margin area around a perimeter of the media access opening, wherein the film is glued to the top surface of the carrier board in the margin area, wherein forming the perforations and structuring the film are performed in the same step with the same method.

3. The method of claim 1, wherein the film is applied to the entire top surface of the carrier board wherein the film, after applying, is structured to be restricted in area to the area of the media access opening on the top surface plus a margin area around a perimeter of the media access opening, wherein the film is glued to the top surface of the carrier board in the margin area, wherein the perforations are at first preformed as blind holes, wherein a plasma etching step is performed later to etch the blind holes until the perforations are through-going through the film.

4. The method of claim 1, wherein a laser is used for structuring the film and for forming the perforations or blind holes.

5. The method of claim 1, further comprising applying a metal layer to an entire surface of the film, forming blind holes at least through the metal layer to form an etch mask for the later forming of perforations by etching.

6. The method of claim 1, wherein a photolithographical structuring is used for forming the perforations and/or structuring the film.

7. The method of claim 1, wherein the mounting the MEMS chip to the carrier board comprises soldering or gluing, and wherein, after mounting thereof, the MEMS chip is sealed to the top surface of the carrier board by a sealing to enclose a volume between MEMS chip, sealing and the carrier board, wherein the perforation area is completely arranged within said volume.

8. The method of claim 1, wherein a package is formed by applying a cap over the MEMS chip and sealing it to the top surface of the carrier board thereby enclosing the MEMS chip and at least another chip necessary for the operation of the MEMS chip between the cap and the carrier board.

9. The method of claim 1, wherein the material of the film includes polymer, polyimide foil, aromatic polyamides polymer foil, or metal foil.

10. The method of claim 1, wherein the film has a thickness of about 1 μm to 10 μm in the area of the media access opening.

11. The method of claim 1, wherein the perforations include a plurality of holes arranged in a regular pattern to provide an area ratio of perforations relative to the perforation area of 30% or more or 50% or more, each of the plurality of holes having a diameter of 2 μm to 10 μm.

12. The method of claim 1, wherein the MEMS chip has a sensing surface, and wherein the MEMS chip is mounted to the carrier board above the film thereby keeping a distance between the perforation area of the film and the sensing surface of at least 10 μm or at least 50 μm.

13. The method of claim 1, wherein the MEMS chip is mounted to the carrier board in a flip-chip assembly.

14. The method of claim 1, wherein the carrier board has a multilayer structure of at least two dielectric layers sandwiching a wiring layer therebetween; wherein the media access opening is structured individually in at least one layer; wherein one of the dielectric layers includes one or more throughgoing holes in the area of the media access opening to provide a filter grid spanning across the media access opening; wherein the diameter of the holes of the filter grid is at least 10 times greater than the diameter of the perforations.

15. A method of manufacturing a MEMS sensor, the MEMS sensor comprising a carrier board with a media access opening, a MEMS chip mounted on the carrier board above the media access opening, and a film applied on a top surface of the carrier board facing the MEMS chip and covering the media access opening, the method comprising the steps of: providing the carrier board having the media access opening; applying the film onto a top surface of the carrier board completely covering the media access opening; providing perforations in a perforation area of the film, the perforation area extending at least partly over an area of the media access opening; mounting the MEMS chip having a surface with a sensing area on the top surface of the carrier board such that a medium to be sensed can reach the sensing area through the media access opening, and wherein the film is applied to the entire top surface of the carrier board, wherein the film, after applying, is structured to be restricted in area to the area of the media access opening on the top surface plus a margin area around a perimeter of the media access opening, wherein the film is glued to the top surface of the carrier board in the margin area, wherein providing the perforations and structuring the film are performed in the same step with the same method.

16. A method of manufacturing a MEMS sensor, the MEMS sensor comprising a carrier board with a media access opening, a MEMS chip mounted on the carrier board above the media access opening, and a film applied on a top surface of the carrier board facing the MEMS chip and covering the media access opening, the method comprising the steps of: providing the carrier board having the media access opening; applying the film onto the top surface of the carrier board completely covering the media access opening; providing perforations in a perforation area of the film, the perforation area extending at least partly over an area of the media access opening; mounting the MEMS chip having a surface with a sensing area on the top surface of the carrier board such that a medium to be sensed can reach the sensing area through the media access opening, and wherein the film is applied to the entire top surface of the carrier board, wherein the film, after applying, is structured to be restricted in area to the area of the media access opening on the top surface plus a margin area around a perimeter of the media access opening, wherein the film is glued to the top surface of the carrier board in the margin area, wherein the perforations are at first preformed as blind holes, and wherein a plasma etching step is performed later to etch the blind holes until the perforations are through-going through the film.

Description

(1) In the following the invention will be explained in more detail with reference to preferred embodiments and the relating figures. The figures are schematically only for better understanding and not drawn to scale. Identical or equivalent parts are referenced by the same reference symbols. Although the examples refer to a MEMS sensor embodied as a MEMS microphone a multitude other MEMS sensors can be embodied similar.

(2) FIG. 1 shows in a cross sectional view a bottom port MEMS microphone according to the invention with a mesh comprising a foil with perforation.

(3) FIG. 2 shows in a cross sectional view a bottom port MEMS microphone according to the invention with a mesh comprising a foil with perforation mounted on a pattern of an adhesive.

(4) FIG. 3 shows in a cross sectional view a MEMS microphone with a mesh comprising a foil with perforation mounted on a pattern of an adhesive with an arrangement of the MEMS chip inverse to that of FIG. 2.

(5) FIG. 4 shows in a cross sectional view a MEMS microphone with a mesh comprising a foil with perforation having a reduced thickness in a central area thereof.

(6) FIGS. 5 to 7 show different shapes of foils as can be used in MEMS microphones according to the invention.

(7) FIGS. 8a to 8h show in a cross sectional view a MEMS microphone after different steps of manufacture.

(8) FIGS. 9d to 9i show in a cross sectional view different process stages of an alternative process of manufacturing a MEMS microphone.

(9) FIG. 1 shows in a cross-sectional view a first embodiment of a MEMS microphone 1 according to the invention. The microphone comprises a MEMS chip MC that is mounted on a carrier board CB such that membrane and back plate BP of the MEMS chip MC are facing a media access opening MO in the carrier board CB. Further, the MEMS microphone 1 comprises an ASIC SC that is a small semiconductor chip that supports and controls the function the MEMS microphone 1.

(10) A mesh MSH that has been formed by perforating a foil directly applied to the top surface of the carrier board CB spans over the media access opening MO and provides a particle filter detaining particles having a diameter greater that the diameter of the holes that form the perforation. A cap CP is glued or soldered onto the carrier board and encloses in its cavity the MEMS chip MC and the ASIC SC. An acoustic seal seals the MEMS chip MC and the AISC SC to the carrier board to provide a front volume for the microphone. Here, the acoustic seal comprises a foil laminated over the chips and structured to uncover the back plate BP.

(11) FIG. 2 shows a similar MEMS microphone 1 with the difference, that the foil is attached to the carrier board by a pattern AP of adhesive. Further, the acoustic seal AS is applied to the bottom edges of the MEMS chip MC only along the perimeter thereof to seal only the MEMC chip to the surface of the carrier board CB.

(12) FIG. 3 shows a similar MEMS microphone with the difference, that the MEMS chip MC and an ASIC SC are attached using an adhesive GL. The back plate is oriented opposite to the carrier board. The MEMS chip can sit on a margin of the foil F with the perforation forming the mesh MSH. The foil is attached by a pattern AP of adhesive. Electrical contacting of MEMS chip and ASIC SC is done via bonding wires.

(13) FIG. 4 shows a MEMS microphone similar to that one shown in FIG. 2 with the difference, that the foil is thinned from the top in a central part. Hence, a margin of the foil F beyond the area of perforation and being in contact with the carrier board has a greater thickness. The mesh MSH comprising the perforation covers the media access opening MO.

(14) FIG. 5 shows in a top view a foil F with a perforation forming a mesh MSH. The foil is structured rectangular to be placed between the bumps.

(15) FIG. 6 shows in a top view a foil F a shape having concavely rounded corners. In the concave recess the bumps for mounting the MEMS chip MC can be placed.

(16) FIG. 7 shows in a top view a foil F a round shape. However, the foil F may have any other shape without departing from the invention.

(17) FIGS. 8a to 8H show in a cross sectional view a MEMS microphone after different steps of manufacture.

(18) FIG. 8a shows a carrier board CB in cross section. It consists of a multilayer laminate with internal electrical connections, vias, and external solder contacts (not shown) for surface mount assembly. Typical materials are HTCC (high temperature co-fired ceramic), LTCC (low temperature co-fired ceramic) or organic laminates (e.g. “FR4”). A media access opening is located in the board CB providing access for external sound pressure to enter the package. As illustrated, this hole may have different size in the various layers. It is also possible, that there's one hole in one layer, and a plurality of smaller holes in another layer. The shape of the holes may be circular or other. Only one unit site is shown. In preferred embodiments of the invention, an arrangement of a plurality of such units formed on the same carrier board (“panel”) is processed in parallel. The singularization (i.e. cutting into individual components) is done after executing several or all steps of the method.

(19) FIG. 8b shows the carrier board after coating with a thin layer of adhesive AL, typically in a thickness between 1 μm to 50 μm, preferably around 10 μm.

(20) FIG. 8c: The adhesive layer AL has been patterned. A very suitable type of adhesive allows for patterning by photolithography. In that case, the layer is exposed by a mask aligner or scanning laser direct imaging (LDI) and then developed. Other patterning methods (e.g. laser ablation) may be employed. As an alternative, the step in FIG. 8b can be omitted and the adhesive is directly applied according to the required pattern, e.g. by dispensing, stamping or printing. Regardless of the method, a high viscosity (>10 Pas) of the adhesive is advantageous. Preferably the adhesive can have thixotropic properties, and is, further preferable, a solid polymer in “B stage” that will melt under heat and pressure before it starts curing. The adhesive chemistry may be based on epoxy, imide, acrylate, or mixtures. Most adequate patterns are closed rings around the media access opening MO. But there may also be additional areas at other locations.

(21) FIG. 8d: A thin foil (typical thickness 1 μm to 10 μm) is laminated onto the adhesive pattern AP across the whole carrier board. For good adhesion, heat and pressure may be applied. Due to its excellent mechanical strength and temperature resistance, polyimide (e.g. Kapton®) is well suited. Other polymers or metal films may also be used. In case of existing features causing a topology to the carrier board surface, the foil F may have corresponding openings. Curing recipes depending on the requirements of the adhesive may be applied.

(22) FIG. 8e: The foil F has been cut so that the remaining foil covering the media access opening MO is held by the adhesive pattern AP, while the surrounding foil is removed easily. This cut may be achieved by a scanning laser, e.g. a UV laser with spot size <100 μm or by an excimer laser.

(23) FIG. 8f: For subsequent flip chip assembly of the MEMS chip MC, solder bumps B are applied onto contact pads on the carrier board CB. This can also be done in an earlier step, but a flat surface facilitates the lamination process. It is also possible to provide the solder bumps on the MEMS chip MC.

(24) FIG. 8g: Now the perforation is applied. An advantageous pattern consists of holes with a diameter of 5 μm arranged in a 7 μm pitch to achieve high acoustic transparency. In general, hole dimensions from 1 μm to 10 μm may be useful. The smallest distance between the holes is 1 μm when measured from edge to edge. Other patterns, pitches and non-circular holes may be used. The total opening ratio should be >30%, preferably around 50% or more with respect to the self-supporting area (perforation area over the media access opening) of the foil. A suitable tool for creating the holes is an excimer laser or short-pulse scanning laser. As an alternative, photolithographical structuring can be applied.

(25) FIG. 8h: A MEMS chip was soldered onto the solder bumps B.

(26) FIGS. 9d to 9i show in a cross sectional view different process stages of an alternative process of manufacturing a MEMS microphone. This method

(27) In this alternative process flow, the laser used for drilling the perforations does not have to drill through the whole polymer foil, but only through a very thin (0.1 μm-1.0 μm) etch mask layer, preferrably a metal layer ML like Ti, Al, or Cr sputtered onto the polymer foil F. By this blind holes BH are produced.

(28) In a next step, the hole pattern is etched through the polymer foil by means of a plasma etching process (e.g. RIE, reactive ion etching, using gases like O.sub.2, CF.sub.4, SF.sub.6 or the like, or mixtures thereof). An anisotropic etching characteristic is preferred.

(29) This method makes it much easier to achieve a proper hole geometry in terms of well defined shape and cross section, and narrow pitch on the one hand. It requires only a very short laser processing time with low thermal influence on the other hand.

(30) It is not necessary to remove the etch mask i.e. the perforated metal layer ML subsequently, because this layer does not contribute significantly to the thickness of the filter mesh MSH. Thus, there is practically no degradation of the acoustical performance of the microphone. Rather, this metal layer ML adds very advantageous features to the assembly, as it helps to shield light and infrared radiation from entering the sound port of the microphone, which otherwise could affect the semiconductor chips contained in the microphone package together with the MEMS chip by generating leakage currents. This would result in noise improves the protection of the component against electromagnetic interference and electrostatic discharge, particularly when the metal layer ML is connected to ground by any suitable means.

(31) The alternative process starts with FIG. 9d showing the same stage like FIG. 8d of the first alternative explained above. The foil F of e.g. 5 μm polyimide has been laminated onto the adhesive pattern AP applied onto the surface of circuit board CB.

(32) Onto the foil F a thin metal layer ML e.g. 200 nm of Ti is deposited e.g. by sputtering. FIG. 9e shows the arrangement at this stage. Further shown are arrows pointing to a cutting line used for structuring the foil F together with the metal layer ML thereupon. FIG. 9f shows the foil F with the metal layer structured e.g. by a circular CO.sub.2 laser cut. The area is now restricted to cover the media access opening MO plus a margin at the perimeter of the opening.

(33) According to FIG. 9g bumps B are applied to the circuit board for later mounting the MEMS chip thereon.

(34) FIG. 9h shows a stage where a pattern of blind holes BH have been drilled by a laser. The blind holes do not go through the whole polymer foil, but only through the very thin structured metal layer ML.

(35) In the next step according to FIG. 9i the perforations PF have been completed by etching through the metal layer ML by means of a plasma etching process (e.g. RIE, reactive ion etching, using gases like O.sub.2, CF.sub.4, SF.sub.6 or the like, or mixtures thereof). Preferably an anisotropic etching characteristic is set.

(36) Thereafter, the manufacture of the MEMS sensor is completed as described before.

(37) In a further alternative embodiment a thicker foil F (of about 10 μm to 50 μm) is used, particularly a foil with an adhesive layer on its lower surface, or with an inherent stickiness. In that case, the steps shown in FIGS. 8b and 8c are not required. For acoustical reasons it may be necessary to locally reduce the thickness in the central area down to 1 μm to 10 μm. It is possible to do this with the same excimer laser. The so produced MEMS sensor is depicted in FIG. 4. In a similar approach local thinning of the foil in a central area is performed from the opposite side of the foil F.

(38) Notwithstanding the number of exemplary embodiments the invention is not restricted to the shown embodiments. The MEMS chip can be of any kind of a sensing chip requiring a media access.

(39) The scope of the invention shall only be limited by the wording of the claims.

LIST OF USED REFERENCE SYMBOLS

(40) 1 MEMS sensor AL adhesive layer AP adhesive pattern AS acoustic seal B bump BH blind hole BP back plate CB carrier board CP cap DL dielectric layer with F foil GL adhesive GRD filter grid MC MEMS chip ML metal layer MO media access opening MSH filter mesh PA perforation area PF perforation SC ASIC